The agency is able to tap into and store data from the cables for up to 30 days so it can be analysed under an operation codenamed Tempora, the Guardian reported.

The Cheltenham-based agency would not comment on intelligence matters but insisted it was "scrupulous" in complying with the law.

The newspaper said there were two principal components to the agency's surveillance programme, called Mastering the Internet and Global Telecoms Exploitation. It claimed the data was shared with the organisation's US counterpart the National Security Agency (NSA).

The information is the latest leak from Mr Snowden, the NSA whistleblower responsible for a string of disclosures about US intelligence operations.

The newspaper claimed Operation Tempora had been running for 18 months and GCHQ and the NSA are consequently able to access vast quantities of communications between entirely innocent people, as well as targeted suspects, including phone calls, the content of email messages, Facebook entries and a user's internet history.

Mr Snowden, who fled the US for Hong Kong after deciding to reveal the NSA's secrets, told the Guardian he wanted to expose "the largest programme of suspicionless surveillance in human history".

"It's not just a US problem. The UK has a huge dog in this fight," he said. "They (GCHQ) are worse than the US."

The newspapers said the documents revealed that by last year GCHQ was handling 600 million "telephone events" each day, had tapped more than 200 fibre-optic cables and was able to process data from at least 46 of them at a time.

A GCHQ spokeswoman said: "We do not comment on intelligence matters. Our intelligence agencies continue to adhere to a rigorous legal compliance regime. GCHQ are scrupulous in their legal compliance."